Air/fuel induction system for spark ignition internal combustion engines, and electromagnetic valves

ABSTRACT

A motor vehicle fuel system operable selectively to supply metered quantities of petrol or LPG in its gaseous state to a carburetter induction passage. The LPG system includes a single stage pressure regulator and an injection valve which may be a digital or an analogue valve. The regulator operates to vary the pressure of gaseous LPG with variations in engine loading by referring that pressure across a single diaphragm to the pressure established in the induction passage downstream of the throttle valve by operation of the engine, while maintaining that regulated gas pressure above that subsisting in the induction passage upstream of the throttle valve. An electronic control system, which may be an analogue control system or a digital control system incorporating a microprocessor having a data store matrix, responds to certain engine operating conditions and controls operation of the injection valve whereby the latter injects metered quantities of gaseous LPG for presentation to the induction passage upstream of the throttle valve in accordance with the requirements of the engine. If a digital injection valve is used, provision is preferably made for smoothing the pulsed output of that valve before it is presented to the induction passage.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to air/fuel induction systems for spark ignitioninternal combustion engines. Such systems comprise an air/fuel inductionpassage with a driver-operable throttle valve therein, and a fuel systemby which gaseous fuel is presented to a location in the inductionpassage, the gaseous fuel being vaporized liquefied petroleum gas (LPG),or a natural gas, such as methane. This invention also relates toelectromagnetic valves which are suitable for use in such systems.

Equipment for fueling motor vehicle spark ignition internal combustionengines with LPG which is available commercially at present is in theform of conversion kits by which an existing motor vehicle sparkignition internal combustion engine installation which includes a petrolsupply system incorporating a carburetter is converted to either an LPGsupply system or a dual fuel system such that the engine can be fueledby either LPG or petrol at the choice of the driver.

Such commercially available LPG fuel system equipment comprises an LPGstorage tank, a vaporizer, a vaporized LPG pressure regulator, and amixer unit. LPG is stored under pressure in liquid form in the tank. Thevaporizer is connected between the interior of the tank and the mixerunit which in turn is fitted to the carburetter so that it communicateswith the carburetter induction passage when the equipment is fitted. Ashut off valve is provided between the vaporizer and the LPG tank whenthe engine installation is converted into a dual fuel system and thatshut off valve is normally closed but is opened automatically by theaction of the driver selecting LPG fuel for fueling the engine. Thevaporizer is adapted to vaporize liquefied petroleum gas fed to it fromthe storage tank, the vaporization being effected by reducing thepressure of the liquefied petroleum gas and bringing it into a heatexchange relationship with water tapped from the engine cooling system,the vaporized LPG being collected in a chamber which is in directcommunication with the carburetter induction passage via the mixer unit.The pressure regulator includes means whereby the pressure of vapour inthat chamber is maintained at a substantially constant near atmosphericpressure. Hence vaporized LPG is drawn into the carburetter inductionpassage via the mixer unit by engine suction and is mixed with the airflow through the induction passage to form the air/fuel mixture that isfed to the engine by the carburetter in the usual way. The amount ofvaporized LPG drawn into the carburetter induction passage is controlledby operation of the mixer unit which is controlled by the demand signalthat comprises engine suction. German Offenlegungsschrift No. 2131804and U.S. Pat. Nos. 3,960,126 and 4,020,810 are concerned with such LPGfuel systems.

Such commercially available LPG fuel system conversion equipment rarelyleads to the true potential of LPG as a low emission fuel beingrealised. The functional performance characteristics of this equipmentare inadequately matched to the requirements of the engine. Even thoughthe engine is tolerant, there are problems on fuel enrichment. Also thelow temperature operation of the equipment is often unsatisfactory.

The equipment is critically dependent upon the mixer units but it hasproved difficult to design mixer units which are sufficiently flexiblein their application for them to be properly matched with the engine'srequirements. Furthermore the mixer units of the LPG conversionequipment can significantly influence the operation of the basic petrolfuel system in an undesirable manner.

The fact that the delivery stage of the part of the equipment thatincludes the vaporizer and pressure regulator operates at a low, nearatmospheric pressure leads to a requirement for large "active" areas butthe resultant design compromise limits accuracy, sensitivity, dynamicresponse and durability of the equipment. Attempts to minimise thesefunctional deficiencies have tended to increase the level of complexityof the mechanically operable pressure regulating equipment. GermanOffenlegungsschrift No. 2131804 and U.S. Pat. No. 3,960,126 disclosesystems which incorporate complicated two stage pressure regulatorswhich operate to vary the pressure of the vaporized fuel within a narrowrange of pressures which are substantially constant and near atmosphericpressure and which comprise a control pressure chamber at ambientpressure which is bounded by two diaphragms and which separates thevaporized fuel from another chamber which is in communication with theengine air/fuel induction passage downstream of the driver-operablethrottle valve. U.S. Pat. No. 4,020,810 discloses the use of aneconomiser valve which is responsive to pressure regions in the engineair/fuel induction passage upstream from and downstream from thedriver-operable throttle valve, and which is effective to modify thepressure in a control pressure chamber of a vaporized fuel pressureregulator to cause it, under certain conditions, to lean out or enrichthe charge supplied from the pressure regulator to a conventional mixingvalve which supplies air according to engine demand.

An object of this invention is to provide fuel system equipment forfueling a motor vehicle spark ignition internal combustion engine withgaseous fuel, such as LPG, the equipment having functional performancecharacteristics which are more adequately matched to engine requirementsthan are those of commercial LPG fuel system equipment currentlyavailable, and being relatively simple from the mechanical viewpoint.

According to one aspect of this invention there is provided an air/fuelinduction system for a spark ignition internal combustion engine, thesystem comprising an air/fuel induction passage with a driver-operablethrottle valve therein, and a fuel system by which gaseous fuel ispresented to a location in the induction passage; the fuel systemcomprising pressure regulating means operable to regulate the pressureof the gaseous fuel and conduit means by which the gaseous fuel isconveyed to said location, the pressure regulating means being adaptedto respond to changes in a depression which is established downstream ofthe throttle valve by operation of the engine and to effect a consequentchange in the regulated pressure of the gaseous fuel such that it variesin the opposite sense to variations of said depression; wherein thepressure regulating means are adapted to maintain the gaseous fuel at apressure higher than that established at said location by operation ofthe engine and comprise a single movable wall which separates twochambers, one of the two chambers containing gaseous fuel at theregulated pressure and the other chamber being in communication withsaid induction passage downstream of said throttle valve, there being aninjection valve in said conduit means operable to effect injection ofmetered quantities of said gaseous fuel whereby said gaseous fuel ispresented to said location, and control means responsive to certainengine operating conditions and operable to control operation of saidinjection valve in accordance with those conditions such that thevolumetric flow rate at which said gaseous fuel is injected forpresentation to said location is matched to the operational requirementsof the engine.

Accordingly, in operation of the present invention, instead ofpresenting fuel vapour to the carburetter induction passage via a mixerunit and relying on engine suction to draw that vapour into theinduction passage via the mixer unit from the delivery chamber whereinthe pressure of the vapour is maintained substantially constant and nearatmospheric pressure by operation of the pressure regulator, we maintainand regulate the pressure of gaseous fuel in the delivery stage at ahigher level which we vary automatically in accordance with, but in theopposite sense to, changes in the engine inlet manifold depression bymeans of a simple single stage pressure regulator, and we inject thatgaseous fuel into the carburetter induction passage, whilst controllingthe opening of the injection valve automatically in accordance withcertain measured engine operating parameters, such as engine speed andengine load. By varying the gaseous fuel injection pressure inaccordance with engine operating conditions as well as controlling theopening of the injection valve in accordance with measured engineoperating conditions, we are better able to match the performance of thefuel system to the requirements of the engine than is possible by merelycontrolling the valve opening. The gaseous fuel fed to the injectionvalve should be a dry gaseous fuel without any liquid phase otherwiseuniform fuel metering cannot reasonably be expected. The range ofcontrol of such an injection valve is limited in practice to such anextent that it is insufficient to adequately match the supply of agaseous fuel to the requirements of the engine over the full range ofengine operation if that gaseous fuel is injected at a constantpressure.

Preferably the pressure regulating means include a pressure relief valveoperable to vent said one chamber into said other chamber and hence tosaid induction passage downstream of said throttle valve, should thepressure in said one chamber exceed a predetermined maximum.

Conveniently the injector valve is controlled electromagnetically bysaid control means. It may be a digital valve, that is to say a valvewhich is so controlled for digital operation, viz. continual valveopening and closing at a controlled frequency and duration of operation.Such a digital valve is advantageous as compared with a simple analoguevalve since friction is not a significant influence on its operation.However the output from a digital injection valve is a pulsed flow ofgaseous fuel and we have not been able to synchronise that pulsed flowwith operation of the engine so that fuel distribution problems followfrom the use of a digital injection valve. With these difficulties inmind we prefer to make provision for smoothing the pulsed output of thedigital injection valve prior to its presentation to said location insaid induction passage. Such provision may include arranging for saidconduit means to communicate with said induction passage through adiffuser arrangement which circumferentially surrounds said inductionpassage at said location and which causes the pulsed output of thedigital injection valve to circulate around the induction passage and bespread circumferentially therearound for presentation to said locationthrough a circumferentially-extending inlet provided. Additionally oralternatively the dimensions of the flow path between the digitalinjection valve and the induction passage may be selected so thatsmoothing of the pulsed output from the digital injector valve iseffected in that flowpath.

The control means may comprise an analogue valve drive circuit adaptedto process an electrical input signal which is the product of electricalsignals indicative of measured values of engine speed and engine load,into a valve driving signal, or a digital drive circuit including amicroprocessor having a data store matrix which is addressed by theelectrical signals indicative of measured values of engine speed andengine load, the microprocessor responding automatically to provide theappropriate valve driving signal derived from data stored in the matrix.

If the injection valve is a digital valve and it is operated at aconstant frequency, the opening pulse width being selected in accordancewith control signals derived from signals indicative of measured valuesof engine speed and engine load, its dynamic range is not sufficientlywide for it to respond to all the control signals that are applied toit. With this problem in mind, we prefer that the digital valve controlmeans are arranged so that, for all operations of the digital valvebelow a predetermined fuel demand requirement of the engine, the valveis operated with a constant opening pulse width at a respectivefrequency selected automatically from within the range of operatingfrequencies of the valve in accordance with the measured values ofengine speed and engine load, whereas for all operations of the valve ator above said predetermined fuel demand requirement, the valve isoperated at maximum frequency, the opening pulse width being selectedautomatically in accordance with the measured values of engine speed andengine load. Controlling the digital injection valve in this way resultsin its having a wider dynamic range than it would have if it was alwaysoperated at maximum frequency.

U.S. Pat. No. 4,141,326 discloses a closed loop fuel control system fora hydrogen fuelled engine which electronically controls fuel delivery tothe engine in response to signals indicative of the engine's operatingparameters and a signal generated by a hydrogen sensor in the exhaustmanifold to maintain the concentration of hydrogen in the exhaust at apredetermined level. The means of metering the flow of hydrogen to theengine comprises an electrically controlled valve of digital or analoguetype. Various analogue circuits for driving the valve are described asis the use of digital electronics.

According to another aspect of this invention there is provided anair/fuel induction system for a spark ignition internal combustionengine, the system comprising an air/fuel induction passage with adriver-operable throttle valve therein, and a fuel system by whichgaseous fuel is presented to a location in the induction passage; thefuel system including conduit means by which the gaseous fuel isconveyed to said location, a digital injection valve in said conduitmeans operable to effect injection of metered quantities of said gaseousfuel whereby said gaseous fuel is presented to said location, andcontrol means responsive to certain engine operating conditions andoperable to control operation of said digital injection valve inaccordance with those conditions such that the volumetric flow rate atwhich said gaseous fuel is injected for presentation to said location ismatched to the operational requirements of the engine; wherein gaseousfuel flow smoothing means are provided within said conduit means betweensaid injection valve and said induction passage whereby the pulsedoutput of the digital injection valve is smoothed prior to itspresentation to said location in said induction passage.

According to another aspect of this invention an electromagnetic digitalvalve is controlled so that, for all operations below a predetermined"mark space ratio" (M.S.R.), it is operated with a constant openingpulse width at a respective frequency selected automatically from withinthe range of operating frequencies of the valve in accordance withcertain external control parameters, whereas, for all operations at orabove said predetermined M.S.R., it is operated at a maximum frequency,the opening pulse width being selected automatically in accordance withthe external control parameters.

According to yet another aspect of this invention, operation of aninternal combustion engine digital electromagnetic fluid fuel injectionvalve is controlled at a frequency which is not synchronised with thespeed of the engine by a control arrangement comprising a microprocessorhaving a data store matrix which is addressed by electrical signalsindicative of measured values of engine speed and engine load, themicroprocessor responding automatically to determine the frequency andpulse width of the digital valve driving signal from data stored in thematrix, the valve having a range of possible operating frequencies andpulse widths.

BRIEF DESCRIPTION OF THE DRAWINGS

An LPG fueling system in which this invention is embodied is describednow by way of example with reference to the accompanying drawings, ofwhich:

FIG. 1 is a schematic diagram of the system which comprises an LPGconversion kit fitted to a spark ignition internal combustion engineinstallation which includes a petrol supply system incorporating acarburetter;

FIG. 2 is a cross-section of the vaporizer/pressure regulating unit ofthe system shown in FIG. 1;

FIG. 3 is a diagram of an analogue electronic control system forcontrolling operation of the injection valve for the system shown inFIG. 1;

FIG. 4 is a cross-section of assembly of a digital injection valve,which serves as the injection valve in the system shown in FIG. 1, andof the upstream end of the carburetter of that system; and

FIG. 5 is a graphical illustration of operation of the digital injectionvalve shown in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a dual fueling system of a motor vehicle formed by fittingan LPG conversion kit to the original petrol system of the sparkignition internal combustion engine 100.

The original petrol system 10 comprises a petrol tank 11, a carburetter12 and piping 13 by which petrol is conveyed from the tank 11 to a floatchamber 14 of the carburetter 12. The carburetter 12 has an air/fuelinduction passage 15 and a driver-operable throttle valve 16 in thepassage 15 whereby the driver controls flow to the inlet manifold of theengine 100 of a mixture of air which is drawn into the induction passage15 through the usual air cleaner by operation of the engine 100 andpetrol which is drawn into the induction passage 15 from the floatchamber 14 through a metering section by operation of the engine 100.

The conversion comprises fitting a solenoid-operated shut-off valve 17into the piping 13, installing an LPG system 18 and connecting its LPGdelivery to the carburetter induction passage 15 adjacent the upstreamend of that passage, and installing an electronic control system 19which includes various transducers which sense respective operatingparameters of the engine 100 and emit control signals derived from thesensed operating parameters.

The LPG system 18 comprises an LPG storage tank 21, a liquid transferpipeline 22 including a solenoid-operated shut-off valve 23, avaporizer/pressure regulator unit 24, piping 25 by which vaporized LPGis conveyed from the unit 24 to an injection valve 26 and means 27 bywhich vaporized LPG injected by the injection valve 26 is directed intothe induction passage 15 of the carburetter 12 at a location upstream ofthe carburetter throat in the sense of the direction of air flow throughthe induction passage 15. It is desirable that the vaporizer iseffective to change the liquid LPG into a dry fuel gas withsubstantially no liquid phase so that the injection valve 26 can operatepredictably to meter the injection of that fuel.

The LPG storage tank 21, the liquid transfer pipeline 22 and the LPGshut-off valve 23 may be conventional and are not described here.

FIG. 2 shows that the vaporizer/pressure regulator unit 24 comprises ahollow casing 28 which is divided internally by a diaphragm 29 into twochambers 31 and 32. The wall of the chamber 32 that faces the diaphragm29 is formed by a casting 33 which is formed with projections that formbaffles projecting into the interior of the chamber 32.

Two spiral passages 34 and 35 are formed substantially concentricallywithin the casting 33. The liquid transfer pipeline 22 is connected intothe spiral passage 34 which leads to the high pressure, or inlet side ofan expansion valve 36 which is mounted in the casting 33, substantiallyat the centre thereof and which controls fluid flow from the spiralpassage 34 into the chamber 32. The spiral passage 35, which has asubstantially larger cross sectional area than the spiral passage 34, isconnected into the cooling water system of the engine 100 and extendsbetween the turns of the spiral passage 34.

The expansion valve 36 comprises a valve body 37 with through passagesformed in it, and an obturating member 38 which slides within a centralone of the passages in the valve body 37. The obturating member 38projects from both ends of the valve body 37 and carries a sealing head39 at its end which communicates with the passage 34. The sealing head39 is urged by a coil spring 41 to seat on the valve body 37 and therebyisolate the passage 34 from the through passages through the valve body37.

A disc 42 is mounted at one end of a rod 43 which extends slidablythrough a bush 44 which is fitted into a central aperture of thediaphragm 29. The disc 42 is within the chamber 32. The rod 43 carries areaction ring 45 within the chamber 32. A coil spring 46 reacts againstthe reaction ring 45 and urges an annular disc 47 against the diaphragm29 so that the central part of the diaphragm 29 is sandwiched betweenthe disc 42 and the annular disc 47 in a gas-tight manner. A third coilspring 48 urges the annular disc 47, and hence the diaphragm 29 towardsthe casting 33.

The chamber 31 is connected by a pipe 49 to the inlet manifold of theengine downstream of the carburetter throttle valve 16. The chamber 32communicates with the piping 25. The spiral passage 35, which isconnected into the engine cooling water circuit, hence comprises onepart of a heat exchange section of the vaporizer/pressure regulator unit24 by which the LPG in the passage 34 and gaseous LPG in the chamber 32are brought into heat exchange with water in the engine cooling watercircuit. The engine cooling water serves as a source of heat forvaporization of the liquid LPG during expansion of that LPG as it flowsfrom the passage 34 to the chamber 32 through the expansion valve 36.

The arrangement of the diaphragm 29 and the third coil spring 48 is suchthat a constant pressure differential is maintained across the diaphragm29. Hence the pressure of gaseous LPG in the chamber 32 is regulated sothat it varies with variations in the inlet manifold depression but inthe opposite sense to the changes in the inlet manifold depression.Thus, for an increasing power requirement from the engine 100, anincreasing pressure gaseous LPG feed to the injection valve 26 isavailable allowing a potential increase in flow of gaseous LPG to theengine. The pressure of gaseous LPG in the chamber 32, and hence in theconduit formed by the piping 25 leading to the injection valve 26, isalso regulated so that it is between the pressure in the storage tank 21and the pressure in the carburetter induction passage 15 upstream of thecarburetter throat. In a practical application of this embodiment ofthis invention, the pressure of gaseous LPG in the chamber 32 ismodulated to 0.8 bar (12 p.s.i.) above the absolute pressure in theengine inlet manifold. Thus as full load of the engine 100, the pressureof gaseous LPG in the chamber 32 is approximately 0.8 bar (12 p.s.i.),and at low loads or idle conditions it is approximately 0.2 bar (3p.s.i.).

Connection of the chamber 31 to the inlet manifold of the engine 100, asdistinct from connecting that chamber to the surrounding atmosphereunder the bonnet of the motor vehicle, avoids undesirable influences onfuel flow to the engine 100 that may follow from changes in the pressureunder the bonnet of the motor vehicle and from the restriction due tothe air cleaner.

The arrangement of the rod 43 that is axially-slidable in the bush 44,the disc 42 and 47, the abutment ring 45, the coil spring 46 and theportion of the surface of the chamber 31 that faces the end of the rod43 in that chamber 31, serves as an over pressure relief valve for thevaporizer/pressure regulator unit 24 which protects that part of thesystem between the expansion valve 36 and the carburetter 12 from beingsubjected to the LPG storage tank pressure in the unlikely event of afailure of the regulator. The bush 44 is permeable by gaseous LPG andserves as a vent path to the chamber 31 for gaseous LPG in the chamber32 when the pressure in the chamber 32 exceeds a predetermined maximumcausing the spring 46 to collapse so that the diaphragm 29 separatesfrom the disc 42. Any gaseous LPG vented from the chamber 32 via thebush 44 is directed to the engine inlet manifold via the chamber 31 andthe pipe 49.

The injection valve 26 is electronically controlled by the controlsystem 19 with the flow of gaseous LPG fluid through it being regulatedby operation of a solenoid. The flow of gaseous LPG to the carburetterinduction passage 15 is regulated by alteration of the effective bleedarea available for the gas flow through the injection valve 26 which isadapted to shut off the flow of gaseous LPG to the carburetter 12 forthe minimum flow condition and is operable to allow flow to thecarburetter induction passage 15 up to a maximum flow. The actualeffective bleed area for the flow of gaseous LPG through the injectionvalve 26 at any instant is determined by the electronic control system19 and is a function of measured engine speed and a load sensingparameter, conveniently the angle of the throttle plate of the throttlevalve 16 although any other engine load parameters such as engine inletmanifold pressure could be used.

FIG. 3 is a block diagram illustrating an analogue electronic controlcircuit 20 which may form part of the electronic control system 19 andby which measured values of engine speed derived from the vehicle enginecontact breaker output N and of engine load derived from a transduceroutput α are used to generate an appropriate solenoid drive signal bywhich the solenoid is energised to operate the injection valve 26 forthe required flow of gaseous LPG.

The engine speed signal N comprises a series of pulses having afrequency which is the measure of engine speed. The signal N isconditioned by being passed through a low pass filter 71, where unwantedhigh frequency components are removed, to a trigger circuit 72 whichproduces a pulse output having a frequency which depends on the signalreceived from the filter 71. The pulse output from the trigger circuit72 is fed to a monostable device 73 which emits a pulse output at thesame frequency as the output from the trigger circuit 72 and having aconstant pulse length which is determined by the setting of a variableresistance 74 with which the monostable device 73 is provided. Theoutput signal from the monostable device 73 is mixed in a mixer 75 witha reference voltage obtained from a tapping of a potentiometer 76whereby that signal is located relative to zero prior to feeding it tothe input of an integrator 77. The output of the integrator 77, which isproportional to the average number of pulses per unit time and hence tothe frequency of the engine speed signal N, is fed to one input of amultiplier 78.

A rotary potentiometer 79, which is mounted on the spindle of thethrottle valve 16, serves as the transducer having the output α which isa measure of engine load. The output signal α is passed through a lowpass filter 81 whereby unwanted high frequency components are removed.The output from the filter 81 is mixed in a mixer 82 with a referencevoltage obtained from a tapping of a potentiometer 83 whereby thatoutput signal is located relative to zero. The signal is then amplifiedin an amplifier 84 to a level at which it is compatible with the outputfrom the integrator 77, and is then fed to another input of themultiplier 78.

The output signals from the integrator 77 and the amplifier 84, whichare indicative respectfully of engine speed and engine load, aremultiplied together by the multiplier 78. The output from the multiplier78 is mixed in a mixer 85 with a reference voltage, which is tapped froma potentiometer 86 which is connected into the multiplier 78, wherebythat output is located relative to zero, and then is fed through abuffer amplifier 87 to an amplifier 88 which generates an analogueoutput signal which is the solenoid drive signal D.

A solenoid drive signal produced by the circuit 20 illustrated in FIG. 3is appropriate for energising an analogue type injection valve. Howeversuch valves have problems due to friction which do not arise if adigital type injection valve is employed. Accordingly we prefer to use adigital type injection valve such as is illustrated in FIG. 4. Whereasthe effective bleed area of an analogue valve is the cross-sectionalarea of the space between the movable valve element and an orifice withwhich it co-operates, the location of that valve element relative to theorifice being a function of the magnitude of the D.C. current (viz. thesolenoid drive signal) by which the solenoid winding is energised tolocate that valve element, the effective bleed area of a digital valve;which continually opens and closes, is related to the frequency of itsopening and the duration of each opening which in turn is determined bythe width of the energising electrical pulse that is the respectivesolenoid drive signal. The analogue output signal from the solenoiddrive signal generating amplifier 88 would be converted into anappropriate pulse solenoid drive signal by an analogue to digitalconverter.

FIG. 4 shows that the digital injection valve comprises a hollow body 51having an internal cavity, an inlet port 52 and an outlet port 53 formedtherein, the inlet and outlet ports 52 and 53 communicating with theinternal cavity on either side of an orifice 54 in a member which spansthe internal cavity. A valve 55 is mounted on a plunger 56 of magneticmaterial which is slidable in the bore of a tubular core 57 which ismounted in the internal cavity coaxially with the orifice 54 and on thesame side of the orifice 54 as the inlet port 52. The valve 55 is urgedto seat in the orifice 54 by a coil spring 58. A solenoid winding 59 iswound around the core 57 and is arranged to unseat the valve 55, againstthe action of the coil spring 58 when it is energised, the plunger 56serving as an armature of the solenoid. The bore of the tubular core 57is closed at its end remote from the orifice 54 by a plug which isrecessed at its end which is nearer to the orifice 54. An O-ring isseated in the recess and serves as a resilient cushion which preventsmetal to metal contact of the plunger 56 with the plug.

The solenoid winding 59 is energised by a pulsed drive signal generatedby the control system 19 so that the valve 55 is repreatedly unseatedand reseated to allow pulses of gaseous LPG to pass through the orifice54 to the outlet port 53. The effective bleed area of the valve 55 for acertain period of operation of that valve 55 is related to the M.S.R. ofthe pulse drive signal, that is to say it is related to the ratio of thetotal of the durations of all solenoid energising pulses during thatperiod to the remainder of that period, and hence is related to thefrequency and pulse width of the pulsed drive signal.

We have not been able to adequately match the supply of gaseous LPGproduced by operation of the digital injection valve 26 to therequirements of the engine 100 by merely altering the pulse width of thesolenoid drive signal whilst operating the solenoid valve at maximumfrequency. In order to increase the dynamic range of operation of thedigital injection valve 26, and thus more adequately match the resultantflow of gaseous LPG to the requirements of the engine 100, theelectronic control system 19 is arranged so that, for all operations ofthe valve 55 below a predetermined fuel demand requirement of the engine100 (say approximately 15% as is illustrated graphically in FIG. 5), thevalve 55 is operated with a constant opening pulse width at a frequencywithin a range of frequencies that increases as the engine fuel demandrequirement increases. This arrangement is illustrated by the portionsof the upper two curves in FIG. 5 to the left hand side of the dottedline. On the other hand for all other operations of the valve 55, thatis to say operations to the right of the dotted line in FIG. 5, thesolenoid drive signal is at maximum frequency and a pulse width whichincreases as the engine fuel demand requirement increases.

Although the electronic control system 19 could incorporate an analoguecontrol circuit similar to the circuit 20 illustrated in FIG. 3 butmodified to provide analogue to digital conversion means for furtherprocessing the output of the multiplier in order to convert that outputto a pulsed solenoid drive signal having a frequency and pulse width forthe sensed engine conditions according to the graphs shown in FIG. 5, weprefer to use a digital control system by which the measured enginespeed and engine load parameter signals are fed each to a respectiveinput terminal of a basic pulse frequency and pulse width memory device61 (see FIG. 1). The device 61 is a microprocessor electronic devicewhich comprises a compact digital store of optimum pulse widths andpulse frequencies for all engine running conditions substantially asillustrated by the graphs in FIG. 5, in the form of a matrix memorystore of injector pulse frequencies and pulse widths tailored to matchthe engine requirements. The microprocessor device 61 is adapted, basedon the two input parameters of engine speed and engine load, to emit anoutput pulsed signal which is the solenoid drive signal appropriate forthe sensed engine condition as illustrated in FIG. 5.

The electronic control system 19 provides other facilities including theselection of the fuel being used, be it petrol or LPG, and thus controlof the opening of the selected one of the petrol solenoid shut-off valve17 and the LPG solenoid shut-off valve 23. The control system 19conveniently includes a switch 62 on the vehicle dash board by which thefuel is selected.

FIG. 4 shows that the outlet port 53 of the digital injection valve 26is connected to the carburetter induction passage 15 through a diffuserarrangement by which the pulsed output of the digital injection valve 26is smoothed before entering the carburetter induction passage 15.

The diffuser arrangement comprises a casing 63 which is fitted onto thatpart of the carburetter body that forms the upper end of the inductionpassage 15 and which receives an air supply duct 64 by which a supply ofclean air for the engine 100 is conveyed to the upstream end of thecarburetter induction passage 15. The diffuser arrangementcircumferentially surrounds the upper end of the induction passage 15.There is a circumferentially continuous gap between the duct 64 and thebody of the carburetter 12. The duct 64 is located relative to theinduction passage 15 by the casing 63 which forms an annular galleryaround the duct 64 and the carburetter body, there being communicationbetween that gallery and the induction passage 15 via the gap. Acylindrical wall of porous material 65 divides the annular gallery intotwo coaxial annular chambers 66 and 67 and hence forms the radiallyinner boundary of the outer annular chamber 66. A port 68 in the side ofthe casing 63 communicates with the outlet port 53 of the digitalinjection valve 26 and connects that outlet port 53 to the outer annularchamber 66. Hence the piping by which the output of the injection valve26 is connected to the diffuser 27 communicates freely with the outerannular chamber 66. Thus the pulsed output of gaseous LPG is conveyedfrom the outlet port 53 to the outer annular chamber 66 around which itcirculates so that it is spread circumferentially therearound. Thegaseous LPG passes through the porous wall 65 into the inner annularchamber 67 and from there into the carburetter induction passage 15through the gap between the carburetter 12 and the air duct 64, wherebyit is presented to the induction passage 15, the gap serving as an inletto the induction passage 15. The length of the flow path for the outputof gaseous LPG from the outlet port 53 of the digital injection valve 26into the outer annular chamber 66 is selected carefully and may be longin order to further the smoothing of the pulsed output from the digitalvalve.

The LPG system 18 may be provided with a drain plug but it may not benecessary as the relatively high pressure flow of LPG may purgeresiduals from the LPG flow passages.

We claim:
 1. An air/fuel induction system for a spark ignition internalcombustion engine, the system comprising an air/fuel induction passagewith a driver-operable throttle valve therein, and a fuel system bywhich gaseous fuel is presented to a location in the induction passage;the fuel system comprising pressure regulating means operable toregulate the pressure of the gaseous fuel and conduit means by which thegaseous fuel is conveyed to said location, the pressure regulating meansbeing adapted to respond to changes in a depression which is establisheddownstream of the throttle valve by operation of the engine and toeffect a consequent change in the regulated pressure of the gaseous fuelsuch that it varies in the opposite sense to variations of saiddepression; wherein the pressure regulating means are adapted tomaintain the gaseous fuel at a pressure higher than that established atsaid location by operation of the engine and comprise a single movablewall which separates two chambers, one of the two chambers containinggaseous fuel at the regulated pressure and the other chamber being incommunication with said induction passage downstream of said throttlevalve, there being an injection valve in said conduit means operable toeffect injection of metered quantities of said gaseous fuel whereby saidgaseous fuel is presented to said location, and control means responsiveto certain engine operating conditions and operable to control operationof said injection valve in accordance with those conditions such thatthe volumetric flow rate at which said gaseous fuel is injected ismatched to the operational requirements of the engine.
 2. An air/fuelinduction system according to claim 1, wherein the pressure regulatingmeans include a pressure relief valve operable to vent said one chamberinto said other chamber and hence to said induction passage downstreamof the throttle valve, should the pressure in said one chamber exceed apredetermined maximum.
 3. An air/fuel induction system according toclaim 2, wherein the injection valve is controlled electromagneticallyby said control means which comprise a digital drive circuit including amicroprocessor having a data store matrix which is addressed byelectrical signals indicative of measured values of engine speed andengine load, the microprocessor responding automatically to provide theappropriate valve driving signal derived from data stored in the matrix.4. An air/fuel induction system according to claim 2, wherein theinjection valve is controlled electromagnetically by said control meansand is a digital valve, said digital valve being operable to continuallyopen and close at a controlled frequency of opening and duration of eachopening so as to control fuel flow in accordance with the frequency ofits opening and the duration of each opening.
 5. An air/fuel inductionsystem according to claim 4, wherein gas flow smoothing means areprovided for smoothing the pulsed output of the digital injection valveprior to its presentation to said location in said induction passage. 6.An air/fuel induction system according to claim 4, wherein the controlmeans comprise a digital drive circuit including a microprocessor havinga data store matrix which is addressed by electrical signals indicativeof measured values of engine speed and engine load, the microprocessorresponding automatically to provide the appropriate valve driving signalderived from data stored in the matrix.
 7. An air/fuel induction systemaccording to claim 6, wherein the digital valve control means arearranged so that, for all operations of the digital valve below apredetermined fuel demand requirement of the engine, the valve isoperated with a constant opening pulse width at a respective frequencyselected automatically from within the range of operating frequencies ofthe valve in accordance with the measured values of engine speed andengine load, whereas for all operations of the valve at or above saidpredetermined fuel demand requirement, the valve is operated at maximumfrequency, the opening pulse width being selected automatically inaccordance with the measured values of engine speed and engine load. 8.An air/fuel induction system according to claim 4, wherein the digitalvalve control means are arranged so that, for all operations of thedigital valve below a predetermined fuel demand requirement of theengine, the valve is operated with a constant opening pulse width at arespective frequency selected automatically within the range ofoperating frequencies of the valve in accordance with measured values ofengine speed and engine load, whereas for all operations of the valve ator above said predetermined fuel demand requirement, the valve isoperated at maximum frequency, the opening pulse width being selectedautomatically in accordance with the measured values of engine speed andengine load.
 9. An air/fuel induction system according to claim 1,wherein the injection valve is controlled electromagnetically by saidcontrol means.
 10. An air/fuel induction system according to claim 9,wherein the control means comprise an analogue valve drive circuitadapted to process an electrical input signal which is the product ofelectrical signals indicative of measured values of engine speed andengine load, into a valve driving signal.
 11. An air/fuel inductionsystem according to claim 9, wherein the control means comprise adigital drive circuit including a microprocessor having a data storematrix which is addressed by electrical signals indicative of measuredvalues of engine speed and engine load, the microprocessor respondingautomatically to provide the appropriate valve driving signal derivedfrom data stored in the matrix.
 12. An air/fuel induction systemaccording to claim 9, wherein the injection valve is a digital valve,said digital valve being operable to continually open and close at acontrolled frequency of opening and duration of each opening so as tocontrol fuel flow in accordance with the frequency of its opening andthe duration of each opening.
 13. An air/fuel induction system accordingto claim 12, wherein gas flow smoothing means are provided for smoothingthe pulsed output of the digital injection valve prior to itspresentation to said location in said induction passage.
 14. An air/fuelinduction system according to claim 12, wherein the control meanscomprise a digital drive circuit including a microprocessor having adata store matrix which is addressed by electrical signals indicative ofmeasured values of engine speed and engine load, the microprocessorresponding automatically to provide the appropriate valve driving signalderived from data stored in the matrix.
 15. An air/fuel induction systemaccording to claim 14, wherein the digital valve control means arearranged so that, for all operations of the digital valve below apredetermined fuel demand requirement of the engine, the valve isoperated with a constant opening pulse width at a respective frequencyselected automatically from within the range of operating frequencies ofthe valve in accordance with the measured values of engine speed andengine load, whereas for all operations of the valve at or above saidpredetermined fuel demand requirement, the valve is operated at maximumfrequency, the opening pulse width being selected automatically inaccordance with the measured values of engine speed and engine load. 16.An air/fuel induction system according to claim 12, wherein the digitalvalve control means are arranged so that, for all operations of thedigital valve below a predetermined fuel demand requirement of theengine, the valve is operated with a constant opening pulse width at arespective frequency selected automatically from within the range ofoperating frequencies of the valve in accordance with measured values ofengine speed and engine load, whereas for all operations of the valve ator above said predetermined fuel demand requirement, the valve isoperated at maximum frequency, the opening pulse width being selectedautomatically in accordance with the measured values of engine speed andengine load.
 17. An air/fuel induction system for a spark ignitioninternal combustion engine, the system comprising an air/fuel inductionpassage with a driver-operable throttle valve therein, and a fuel systemby which gaseous fuel is presented to a location in the inductionpassage; the fuel system including conduit means by which the gaseousfuel is conveyed to said location, a digital injection valve in saidconduit means operable to effect injection of metered quantities of saidgaseous fuel whereby said gaseous fuel is presented to said location,said digital valve being operable to continually open and close at acontrolled frequency of opening and duration of each opening so as tocontrol fuel flow in accordance with the frequency of its opening andthe duration of each opening, and control means responsive to certainengine operating conditions and operable to control operation of saiddigital injection valve in accordance with those conditions such thatthe volumetric flow rate at which said gaseous fuel is injected forpresentation to said location is matched to the operational requirementsof the engine; wherein gaseous fuel flow diffusion means are providedwithin said conduit means between said injection valve and saidinduction passage whereby the pulsed output of the digital injectionvalve is smoothed prior to its presentation to said location in saidinjection passage.